In the prior art, when the multi-split air conditioner is in heating, because of the lower flow velocity of the refrigerant in the main gas pipe and in the gas pipe of the non-operating indoor unit, oil is apt to be accumulated in these segments of gas pipes, which will result in a hidden trouble of oil lack in the compressor, thereby causing insufficient lubrication. The compressor may operate abnormally if the situation is mild, and the compressor will be damaged if the situation is severe. In order to ensure the reliability of the air conditioner, oil return process must be performed after a period of time of heating operation.
In the prior art, the oil return method for multi-split air conditioner in heating commonly comprises steps of: switching the four-way valve to the refrigeration mode; returning the lubricating oil accumulated from the segments of gas pipes with the aid of the liquid refrigerant. With this kind of oil return method, during the oil return period, the indoor unit acts as an evaporator, meanwhile the indoor unit stops heating, and after the oil is returned, the air conditioner remains in a low pressure state for a period of time (namely, it takes a period of time for the air conditioner to establish the high pressure state). Therefore, during the oil return period, and at the early stage of heating before the high pressure state is established, the heating operation cannot be performed efficiently. Accordingly, the comfort in use of the multi-split air conditioner in heating will be affected due to this kind of oil return method. In order to solve this problem, some multi-split air conditioners adopt an oil return method of increasing the operation frequency of the compressor and increasing the step number of the throttling elements of the indoor units without the need of switching the four-way valve. This kind of oil return method will reduce influences of the oil return process on the comfort in use of the air conditioner in heating, but will more greatly affect the running of the multi-split air conditioner during oil return process.